The present invention relates generally to Coriolis-type mass flowmeters and in particular to mass flowmeters employing oscillating conduits.
In response to the need to measure the quantity of material being delivered through pipelines, numerous types of flowmeters have evolved from a variety of design principles. One of the more widely used types of flowmeters is based on volumetric flow. Some designs employ turbines in the flow line; others operate on a resilient vane principle. Of course, volumetric flowmeters are at best inaccurate in determining the quantity of material delivered, where the density of the material varies with temperature or feedstock or where the fluid being pumped through the pipe line is polyphase such as a slurry or where the fluid is non-Newtonian such as mayonnaise and other food products. In the petroleum field, so called "custody transfer" requires accurate measurement of the exact amount of oil or gasoline being transferred through the pipeline. The higher the price of oil, the more costly the inaccuracy of the flow measurement. In addition, chemical reactions, which are in effect mass reactions where proportions are critical, may be poorly served by volumetric flowmeters.
These problems are supposed to be solved by mass flowmeters which provide a much more direct indication of the quantity of material--down theoretically to the molecular level--which is being transferred through the pipeline. Measurement of mass in a moving stream requires applying a force to the stream and detecting and measuring some consequence of the resulting acceleration.
The present invention is concerned with improvements in one type of direct mass measuring flowmeter referred to in the art as a Coriolis effect flowmeter. Coriolis forces are exhibited in the radial movement of mass on a rotating surface. Imagine a planar surface rotating at constant angular velocity about an axis perpendicularly intersecting the surface. A mass travelling at what appears to be a constant linear speed radially outward on the surface actually speeds up in the tangential direction. The change in velocity implies that the mass has been accelerated. The acceleration of the mass generates a reaction force in the plane of rotation perpendicular to the instantaneous radial movement of the mass. In vector terminology, the Coriolis force vector is the cross-product of the angular velocity vector (parallel to the rotational axis) and the velocity vector of the mass in the direction of its travel with respect to the axis of rotation (e.g., radial). Consider the mass as a person walking on a turntable and the reaction force will be manifested as a listing of the individual to one side to compensate for acceleration.
The applicability of the Coriolis effect to mass flow measurement was recognized long ago. If a pipe is rotated about a pivot axis orthogonal to the pipe, the material flowing through the pipe is a radially travelling mass which, therefore, experiences acceleration. The Coriolis reaction force shows up as a deflection or offset of the pipe in the direction of the Coriolis force vector in the plane of rotation.
Mass flowmeters in the prior art which induce a Coriolis force by rotation fall into two categories: continuously rotating and oscillating. The principal functional difference between these two types is that the oscillating version, unlike the continuously rotating one, has periodically (i.e., usually sinusoidally) varying angular velocity producing, as a result, a continuously varying level of Coriolis force. In addition, a major difficulty in oscillatory systems is that the effect of the Coriolis force is relatively small compared not only to the drive force but also to extraneous vibrations. On the other hand, an oscillatory system can employ the bending resiliency of the pipe itself as a hinge or pivot point for oscillation and thus obviate separate rotary or flexible joints.
Some of the remaining problems with prior art Coriolis effect mass flowmeters are that they are too sensitive to extraneous vibration, require precise balancing of the conduit sections undergoing oscillation, consume too much axial length on the pipeline and produce undue stress and resulting fatigue in the conduit at the flexure point or fail to provide adequate mechanical ground at the flexure point of the oscillating conduit.